1. Field of the Invention
The invention in general relates to electronic test probes for passing a test signal from a electronic circuit element to be tested to an oscilloscope or other electronic measurement device, and more particularly to such a probe that passes the test signal with high signal integrity over a high band width.
2. Statement of the Problem
Electronic test probes are commonly used to pass analog test signals from a circuit under test to an oscilloscope or other electrical or electronic test instrument. Such an electronic probe must be capable of passing an electrical signal on a node or pin of the circuit under test tests to the test instrument without distoting it, i.e. with high signal integrity. Further, it should not apply any voltage or current to the circuit under test.
Early probes consisted of carefully designed passive circuits with high impedance inputs and with an output impedance comparable to the input impedance of the test instrument. Such passive probes simply passed a signal from the circuit under test to the test instrument.
Present-day electronic circuits operate over frequencies from DC to several gigahertz. Thus, test probes capable of being used with a wide variety of circuits must be able to provide high signal integrity over a wide band width of frequencies. Thus, state-of-the-art test probes are active probes, that is, probes with active circuit elements, such as transistors, driven by a probe power source. Such probes, though small enough to be easily manipulated by hand, are highly sophisticated instruments that may cost several thousand dollars.
The art of test probes that pass analog signals to a test instrument should be distinguished from the digital test equipment art. In the latter art high signal integrity is not a significant goal, since digital test instruments only need to detect the rise or fall of a digital signal.
Integrated and hybrid circuits are becoming both more complex and smaller, leading to ever higher numbers of package leads crowded into less and less space, that is, the leads are becoming extremely dense with very tight pitches. The art has responded to this requirement by producing large numbers of gadgets designed to interlace with circuit packages, such as plastic quad flat packs (PQFP). These gadgets provide an interconnect between the dense array of output pins on a circuit package to a less dense array of outputs that can be more easily contacted manually with a hand held probe.
Another solution is a probe multiplexing unit, such as the HP 54300A probe multiplexer made by Hewlett Packard Company. A number of probes can be connected to the back of this multiplexing unit, and the probe to be connected to the output can be selected by a mechanical switch. This unit is capable of multiplexing eight probes and permits switching back and forth between a few circuit points without manually resetting a probe. The system still requires mechanical setup of each probe, and is relatively expensive, since it requires a multiplicity of probes. Moreover, the unit is a thirty pound box requiring about two square feet of volume and does not lend itself to rapidly probing a large number of microcircuits.
All of the above state-of-the-art electronic probe systems have many drawbacks. It is time consuming to measure signals one at a time by hand. Further, the interconnect methods leave uncertainties in the ground and signal path lengths that can significantly affect the signals under test. Moreover, even if the path lengths are known, the existence of a long signal path from package, through connector, through probe input cable, through probe, through probe output cable, to test instrument can itself result in measurement inaccuracies. Thus the state-of-the-art electronic test probe system is subject to coupling errors, overshoot errors, risetime errors, timing errors in general and other inaccuracies.
3. Solution to the problem
The present invention solves the above problems by providing an electronic test probe having a multiplexer in the probe head. The multiplexer is programmable, allowing the user to selectably connect any one of a large number of probe inputs to any one of a number of probe outputs. In the preferred embodiment, any two of several hundred inputs can be connected to two outputs. The multiplexer is implemented on an integrated circuit (IC) chip.
The combination of a programmable multiplexer implemented on an integrated circuit chip permits hundreds of inputs to be alternatively connected to the probe output, with no manual manipulation of the probe itself, and the elimination of all the potentialities for error that go along with such manipulation. For each connection on the integrated circuit, the signal and ground paths are both well-defined and short. This eliminates all the inaccuracies due to signal path uncertainties and length.
The integrated circuit chip is designed so that any number of chips can be daisy-chained together in the probe head so that different probe heads accommodating different packages on the market can be economically designed and manufactured. Herein, "daisy-chained" means that the outputs of individual components, such as multiplexers or chips, can be connected to a single line, at any point in the line, which line then forms the common output for all such components. To design and build a separate integrated circuit for each package on the market would be exorbitantly expensive. With the chip of the invention, just a few chips can accommodate a large percentage of the available circuit packages.
The invention also includes a programmable, selectable gain amplifier within the chip. Thus both the input and the gain for each input are selectable, while still maintaining the advantages described above.
The invention further includes on-chip input dividers, so that the signals can be attenuated without losing any of the above advantages. There are also on-chip spark gaps and diode clamping so that ESD events do not interfere with the high band width, high signal integrity in the probe output. The on-chip circuit fully compensates for the ESD devices so the ESD devices themselves also do not interfere with the high band width, high signal integrity in the probe output.